This invention relates to magnetic recording media, and more specifically to magnetic recording media suitable for use in high-density recording.
Needle-like .gamma.-Fe.sub.2 O.sub.3 has conventionally been employed most extensively in magnetic recording media. However, due to the low coercive force (iHc) of .gamma.-Fe.sub.2 O.sub.3 which generally ranges from 300 to 350 Oe, it has been impossible to increase the recording density using 65 -Fe.sub.2 O.sub.3 alone.
In view of the circumstance, it has been attempted to increase the iHc of acicular .gamma.-Fe.sub.2 O.sub.3 particles to about 700 Oe or so by applying Co on the surface of the .gamma.-Fe.sub.2 O.sub.3 particles prior to their use. Even in this attempt, it is infeasible to increase the iHc beyond the above-mentioned level.
Meanwhile, so-called metallic tapes have been known, which are prepared by dispersing fine particles of magnetic iron powder in a resinous binder and coating the thus-prepared coating formulation onto surfaes of substrates. They have the a merit that they permit magnetic recordings of higher density than conventional magnetic iron oxide, .gamma.-Fe.sub.2 O.sub.3 , but, on the other hand, they are not only difficult to handle because they undergo ready oxidation by oxygen present in the air and have an extremely high potential danger of explosion in the course of preparing the magnetic iron powder or magnetic coating formulation, but they are also accompanied with a drawback that their characteristics as magnetic recording media have poor long-term stability.
Furthermore, it has recently been attempted to prepare coating formulations using .gamma.-Fe.sub.2 O.sub.3, the particle lengths of which .gamma.-Fe.sub.2 O.sub.3 have been shortened as much as feasible, and applying such coating formulations onto substrates to provide magnetic recording media of high density.
However, as the particle lengths of needle-like ferromagnetic particles are shortened, a limitation will appear as to reproduction output in the high-frequency region and their electromagnetic characteristics in the low-frequency region will be deteriorate because of the influence of demagnetizing fields, leading to another problem that the usable frequency region will be narrowed down.
With a view toward solving these problems, the present inventors have already proposed a high-density magnetic recording medium in which ferromagnetic particles such as needle-like .gamma.-Fe.sub.2 O.sub.3 particles or Co-coated .gamma.-Fe.sub.2 O.sub.3 particles and fine magentic particles of the Ba-ferrite type are both contained at predetermined proportions (see, Japanese Unexamined Pat. Publication No. 212623/1982, published Dec. 27, 1982). In the above proposal, each piece of the fine magnetic particles of the Ba-ferrite type serves as a minute recording element capable of functioning as a magnet of a single magnetic domain, and, besides, its iHc is as high as 600 to 2,000 Oe, thereby making high-density magnetic recording feasible.
A subsequent investigation by the present inventors has, however, found that, since fine magnetic particles of the Ba-ferrite type are crystals of hexagonal plate-like configurations and have easy axes of magnetization in directions vertical to their planes, the fine particles tend to agglomerate one over another because of a magnetic attraction produced between each plate-like particle and its adjacent plate-like paticle. This means that, when fine magnetic paticles of the Ba-ferrite type are dispersed together with .gamma.-Fe.sub.2 O.sub.3 particles in a resinous binder upon preparation of a magnetic coating formulation, the dispersion may not always be uniform and may thus lack stability and, when applied on substrates, the non-uniformity of the dispersion may inconveniently affect deleteriously high-density magnetic recording and, in addition, become a cause for deteriorated anti-noise characteristics.